What is the compression ratio of a Prius v engine?

Discussion in 'Prius v Technical Discussion' started by Eric "v", Mar 3, 2015.

  1. Eric "v"

    Eric "v" Member

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    I'm assuming the compression ratio is over 10:1 but with theAtkinson cycle engine we have it may be lower due to the late closing of intake valves to achieve an Atkinson cycle effect.

    Anyone know where to find factory info? Or cylinder PSI info for compression testing?
     
  2. css28

    css28 Senior Member

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    Compression pressure: 813 kPa (8.3 kgf/cm^2, 118 psi)
    Minimum pressure: 617 kPa (6.3 kgf/cm^2, 89.5 psi)
    Difference between each cylinder: 100 kPa (1.0 kgf/cm^2, 14.5 psi)
     
  3. rjparker

    rjparker Senior Member

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    There is something of an ongoing debate about the compression ratio. Some suggest it is variable from ~9/1 to 13/1 while others argue the specs indicate the "expansion ratio" is 13/1 but that is not technically the compression ratio. In any case the engines uses variable valve timing to increase the volume of fuel mixture in the cylinder and uses variable spark timing to prevent knocking. Then there is a discussion about use of higher octane gas to get better mpg. I strongly believe better octane will give better mpg and also know that different brands have different "87" octanes in reality. One may be selling an 87 that is really 86 while another is meeting the 87 octane spec. I have even seen a major company increase their octane when an expensive advertising campaign was occurring.

    See What grade of gas is best? | PriusChat
     
    #3 rjparker, Mar 3, 2015
    Last edited: May 5, 2015
  4. Eric "v"

    Eric "v" Member

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    I asked so I would know I'm not wasting money on 89 (RON) octane. I do not trust 87 octane to always be that high even though I almost always use Chevron gas, a reliable brand.

    I used to use 91 octane in my RAV 4 V6 except for 87 octane in hot summers here in 'Vegas, or at high altitudes like Denver, when the air is much "thinner" and high compression power gains can only be had with turbos and superchargers.
     
    #4 Eric "v", May 5, 2015
    Last edited: May 11, 2015
  5. jzchen

    jzchen Senior Member

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    Manuals usually state whether there is a benefit for higher ocatane fuel. Why risk knock at any time? Air might be thinner but everything is hotter in the summer. Unless you can find me a formula regarding thinner hotter air and autoignition I would stick with the original "protect my engine" theory. Complicated stuff this internal combustion....
     
  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    In the US, no gen 1, gen 2, gen 3, v, or PHV engine meeds more than 87 when new. Carbon buildup when ancient may alter that.

    Some c owners claim improbement with higher octane, I have never owned a c.

    Above 4500 feet, 85 octane is fine.
     
  7. jzchen

    jzchen Senior Member

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    It is difficult to "feel" what is going on. I'm honestly not sure many know exactly what 87 is when it is pumped out of the pump. Is the fuel container underground clean? Is there no contamination? Do they know that you could rate water with regards to octane? Driving style definitely makes a difference. The smaller engines work harder. Many, many factors to deal with. Told my wife to use 89 just in case for her C-MAX Energi...

    (My wife just filled my mother's '92 190E 2.6 with 89. I opened my manual to see what recourse I could follow: she definitely didn't follow "have the tank filled only partially with unleaded regular and fill up with premium unleaded as soon as possible", I'm not sure if she followed "avoid full throttle driving and abrupt acceleration," I advised her that the manual says "do not exceed an engine speed of 3000 rpm, if the vehicle is loaded with a light load such as two persons and no luggage." I'm not sure she knows/cares what 3000 rpm is or how to tell how to keep under that. She definitely didn't ask me. It also says "do not exceed 2/3 of maximum accelerator pedal position, if the vehicle is fully loaded or operating in mountainous terrain.") I worked my butt off to keep all the cars in the household in shape, especially those that we are currently using......
     
  8. rjparker

    rjparker Senior Member

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    A 2013 Prius v that the op asked about is rated for 87 octane and will compensate for higher or lower altitude, load or octane automatically and effectively. Yes, some 20 year old performance cars did not have knock sensors and required premium, and a Prius and most other new cars will provide marginally better mpg with a higher octane because of the timing compensations, but it's not worth the extra cost.

    In the old days, some no name brands of gas might not include additives in their regular grade but even then, there was little guarantee the distribution facility ("product terminal") had a quality fuel injection cleaning additive system in the first place. Today even the lowest cost "Costco" style regular gas comes with detergent additives, which means you can go for the best price. One caution I observe is to avoid filling at a station while the tanker truck is delivering fuel. Any water or dirt in the station tank can get mixed at that time but normally settles to the bottom below the pump's intake.
     
    #8 rjparker, May 6, 2015
    Last edited: May 6, 2015
  9. jzchen

    jzchen Senior Member

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    I do not want a knock sensor to be the determining factor for my engine, as well as anyone else's engine for that matter. Knock sensors detect knock, meaning a knocking event occurred in the first place. This event I never would want, it is equivalent to damage to your engine. (What I should do is explain to the family how knock is the equivalent to inducing a heart murmur in a human. Then maybe they would listen more carefully to me.) That said, I believe I read somewhere there are other sensors available that prevent engine harm/knock. I just don't have much knowledge about those other methods....

    I tried to figure out whether hot weather or high altitude would allow lower octane fuel. I searched for information using Google. I saw one formula relating exposed temperature, heat flux, and autoignition, but could not determine from this whether it is true that higher altitudes for example, means you could use lesser octane fuel. Some said yes, but some countered that variable valve timing puts this theory out. (In my mind it's a wash, but I didn't read through all the articles.)

    BTW- Last time I was at Costco, I noticed, (as someone on a forum pointed out,) that they have reached Top Tier status. I haven't seen the same for Sam's Club though, but I haven't been to Sam's Club gas for a little while now...
     
    #9 jzchen, May 6, 2015
    Last edited: May 6, 2015
  10. css28

    css28 Senior Member

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    jzchen,
    You're overthinking it.

    Really overthinking it.
     
  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I suspect there is not an engine designed in the last 20 years that does not use a knock sensor to adjust ignition timing.
     
  12. Eric "v"

    Eric "v" Member

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    css28, could you PULEZZE convert those metric numbers into "English" measurements?
     
  13. jzchen

    jzchen Senior Member

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  14. Eric "v"

    Eric "v" Member

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    Thanks jz. My maths-challenged brain can now comprehend the simpler version of a 13:1 ratio.

    But yes, only a automotive combustion engine engineer can explain the valve control/compression ratio intricacies Toyota uses to achieve its Atkinson cycle efficiencies.

    It's interesting that variable valve timing can replicate the Atkinson cycle without the strange piston gymnastics used in the original concept.
     
  15. CrazyLee

    CrazyLee Member

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    Knock sensors are very sensitive gadgets. You can't hear the knock but the sensor can. So don't worry about it.

    The 13:1 compression ratio is interesting. A diesel has to have 16:1 compression to run at all.
    I would think if the compression really was 13:1 it would knock all the time. Maybe have lots of pre-ignition at shut off. +
    By advancing the valve timing allows some blow back into the intake manifold that reduces some compression.
    I think that is why the MAF has to be cleaned sometimes.
    I can see when the Atkinson cycle starts. When accelerating the MPG's are low. When you lift the throttle the mpg's get better but not the best. Lift the throttle just a little bit more and the mpg's will double, meaning the Atkinson cycle has started. My mpg will go from 25 to 50 mpg.
     
  16. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    It does not really have a compression ratio of 13:1, it has an expansion ratio of 13:1.

    The compression ratio is modified by the fact that the intake valves stay open for 30 degrees of the compression stroke, and is nearer to 8:1.
     
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